A quantum dot (QD) is a semiconductor particle with a nano size. Since the QD has the very small size, a quantum confinement effect is shown. The quantum confinement effect means a phenomenon in which when an object is decreased to a nano size or less, a band gap of the object is increased. Accordingly, when light with a wavelength having larger energy than the band gap of the quantum dot is incident to the quantum dot, the quantum dot is in an excited state by absorbing the light and falls to a ground state while emitting the light having a predetermined wavelength. The wavelength of the emitted light has a value corresponding to the band gap. Since light emission characteristic due to the quantum confinement effect varies according to the size, the composition, and the like of the quantum dot, the quantum dot has been variously used in various light emitting devices and electronic devices by adjusting the size, the composition, and the like thereof.
A quantum dot liquid crystal display (QD-LCD) having excellent color reproducibility may be produced by using the quantum dot. The quantum dot is used as a phosphor in the display device, and in order to optically coupling a light source and the quantum dot, a sheet (hereinafter, referred to as a quantum dot sheet) prepared by distributing the quantum dot in a resin is used.
The quantum dot is formed of a core, a shell, and a ligand. The core serves to emit light, and the shell is formed by covering the core on the surface of the core to protect the core. In addition, the ligand is formed by covering the shell so that the quantum dot is distributed in an organic solvent well. Generally, the ligand is formed of an organic material, such as, trioctylphosphine, oleic acid, and amine.
However, even though the quantum dot itself is made of the semiconductor material, when the organic ligand is bonded, the bonded organic ligand acts as an insulating barrier layer, and thus, there is a limitation in that the electrical conductivity of the quantum dot is deteriorated. Further, the organic ligand is a polymer material, and van der Waals force acts as attraction force between the adjacent organic ligands. The attraction force causes aggregation, condensation, or the like of the quantum dots, and thus, there is a problem in that the light emission efficiency of the QD-LCD is deteriorated.
In the quantum dot sheet, a plurality of quantum dots and a light scattering agent are distributed in the resin.
The resin serves to protect the quantum dots from an external shock and environment, and distribute and fix the quantum dots and the light scattering agent. The resin serves as a primary element for determining reliability, cost, and performance of the quantum dot sheet. The resin requires a high refractive index, characteristics of blocking oxygen and moisture, and excellent heat resistance in order to increase light extraction efficiency of the quantum dots and secure the reliability of the quantum dot sheet.
A general resin used in the quantum dot sheet in the related art is an epoxy resin. The epoxy resin which is low in cost and is a sealing material of an optical member has an excellent characteristic, but has low force of blocking the oxygen and the moisture and a yellowing phenomenon at a high temperature, and as a result, there is a limit that the epoxy resin is insufficient to be applied to a high-efficiency light source device such as a quantum dot liquid crystal display (QD-LCD).
In the quantum dot liquid crystal display (QD-LCD), various methods for combining the quantum dots and the light source may be provided, but an on-surface method for manufacturing a back light unit (BLU) by placing the quantum dot sheet on a light guide plate is primarily used.
The back light unit in the related art has a structure in which the quantum dot sheet is stacked on the top of the light guide plate. Light emitted from the light source is diffused in the light guide plate and the light diffused in the light guide plate is incident in the quantum dot sheet toward the top. While the incident light passes through the quantum dots distributed on the quantum dot sheet, a wavelength of the light is converted. It is advantageous in that the quantum dot sheet may be applied to a display module while not changing a thickness or a structure of a liquid crystal display in the related art.
However, in the back light unit in the related art, when the quantum dot sheet is stacked by the on-surface method, a base film of the quantum dot sheet is in contact with the top of the light guide plate. In general, a refractive index of the light guide plate is approximately in the range of about 1.4 to 1.5 and the base film of the quantum dot sheet also has a refractive index in a range similar thereto. A refractive index difference between the light guide plate and a base sheet is slight, and as a result, light incident in the light guide plate is transmitted toward the base film while a full-reflection phenomenon cannot smoothly occur. Therefore, there is a problem that the incident light cannot be uniformly diffused in the light guide plate and the light is intensively emitted around a light receiving unit of the light guide plate. As a result, the quantum dot sheet cannot uniformly receive the light from the light guide plate. Therefore, the back light unit in the related art has a limit that even though a quantum dot having excellent color reproducibility is used as a fluorescent substance, it has low light conversion efficiency.
Further, the back light unit in the related art is a structure in which the quantum dot sheet is simply stacked on the top of the light guide plate. The quantum dot sheet which has a thin film shape is vulnerable to heat generated when the light is converted at the quantum dot, and the like. Accordingly, when only the quantum dot sheet is just stacked on the top of the light guide plate, there is a problem that the quantum dot sheet curls or wrinkles as the back light unit operates occurs.
The back light unit including the quantum dot sheet which curls or wrinkles cannot supply uniform light to a liquid crystal panel. Furthermore, a liquid crystal display device including the quantum dot sheet and the back light unit has a problem that a color difference error occurs.